Deleting user bug when user changes e-mailadress


When using Cp and Membership pro I have a problem that some members are not deleted
when they changed their e-mailadres.
This is very annoying with growing big groups. The connection to the inactive groups list on mailchimp is also not correctly then.

Any suggestions? Or could I prevent with a hack to alter e-mail adresses by the users?
Just like the username?

Ciao Valentijn